In this paper, the considered system corresponds to a specific equipment proposed for leasing. This equipment is leased to several lessees who use it under various working conditions. The aims of this paper consist in determining optimal maintenance plans which minimize maintenance costs. Two maintenance policies are compared : systematic policy (imperfect preventive maintenance actions are performed, with the same effectiveness factor, after each mission) and “sporadic” policy (imperfect preventive maintenance action can be performed after a mission with its optimal effectiveness factor).